Revealing the Smallest Particles (Science Advent 5)

Day 5

The tracks of electrically charged particles passing through a bubble chamber of superheated liquid hydrogen at CERN. The tightly coiled spirals are electrons or positrons (which coil in opposite directions), while the straighter lines are more massive particles. The V-shaped intersections come from a neutral (invisible) particle decaying into two oppositely charged new particles.

We can’t see subatomic particles directly. Visible light has wavelengths far too large; gamma-ray light is small enough, but carries sufficient energy to strongly affect the particles we’re trying to study. One way to see particles indirectly is a bubble chamber, a device once commonly used at the big particle-physics labs, and still used for hunting for dark matter (in a slightly different form). The major type of bubble chamber is filled with some liquid with a low boiling point, often hydrogen. (Hydrogen is good because it doesn’t have any neutrons, which introduce another layer of complexity in interactions.) Just as water in a pressure cooker can be heated well above boiling point without actually starting to boil, the liquid hydrogen in a bubble chamber is superheated above the temperature where it would ordinarily vaporize. (Despite the name, superheating in this case is still cryogenically cold!) The whole chamber is subjected to a strong magnetic field.

When an electrically charged particle enters the chamber, they disturb the liquid, boiling it along the path they travel—creating the bubbles that give bubble chambers their name. The magnetic field causes the particles’ trajectories to bend proportionally to their momentum: low-mass particles like electrons and positrons (antimatter partners of electrons) end up moving in tightly coiled spirals, while heavier particles are harder to deflect and travel in straighter lines. Additionally, negatively charged particles will spiral in the opposite direction from positively charged particles. Neutral particles don’t show up, but some of them are unstable and decay into pairs of positive and negative new particles, creating the distinctive sideways “V” pattern in the image above. This means that, with some effort, bubble chambers can be used to identify particles only from the paths they make.